Strengthening rear diff mounting - L series

AndrewT · Post by **AndrewT** » Mon Jun 15, 2009 12:15 pm

In a little while my L series is going to be running RWD most of the time when driving on tarmac (I will be able to engage the front wheels when going offroad).
I'll be running a 4.111 clutch LSD at the rear and a stock EJ20G turbo motor.

Most people who run EJ20G's have strenthened the rear moustache bar by cutting the two mounting points off a second one and welding it to the first one, which doubles the thickness, this usually stops the problem of the bar itself twisting.

I'm interested in going a step or two further than this to ensure the rear end is nice and strong and will last in this configuration reasonably long term.

Already have a few ideas for extra braces etc but just wondering if anyone else can make some more suggestions.

(Note - At this stage just looking for suggestions on strengthing the mounting system etc. Leave the diff itself and axels, cvs etc out of it for now.)

cheers!

bluesteel · Post by **bluesteel** » Mon Jun 15, 2009 12:19 pm

ive run a peice of angle iron along the moustache bar to stop that flexing, but welding anything along its length will stiffen it up

also i have filled the front bush up with sikaflex so now its a solid bush, and stops that flexing a lot too (theres usually a fair ol bit of movement in that one too)

brumbyrunner · Post by **brumbyrunner** » Mon Jun 15, 2009 2:40 pm

This is what I came up with.

Rear mount

Front mount

AndrewT · Post by **AndrewT** » Mon Jun 15, 2009 3:29 pm

That certainly looks incredibly solid, but wouldn't there be a fair bit of vibration transferred to the body?
The stock mounts are on rubber at all three points (the one at the front of the diff, and both ends of the moustache bar).

Point · Post by **Point** » Mon Jun 15, 2009 3:55 pm

you could possibly make something similar to Brumbyrunner's rear mount, but use the short bracket out of a later forrie or lib for the rubber mounts

edit...piccy....

not exactly sure which models were like this

AndrewT · Post by **AndrewT** » Mon Jun 15, 2009 4:40 pm

ah-hah. now we're talking. I did think of using the rear end from a EJ based car (bound to be stronger) but hadn't found any detailed photos of how they are setup - Never had an AWD Lib so I've never actually dealt with one of them before. Looks pretty good...very similar to the L moustache bar but so much shorter so I can't see that really twisting. Never heard of anyone with a Lib or wrx having that kind of issue even with some guys pushing rediculous housepower.

stamp_licker · Post by **stamp_licker** » Mon Jun 15, 2009 7:08 pm

A couple of attached pics that give you another option.This is not on my L.

Subafury · Post by **Subafury** » Mon Jun 15, 2009 8:02 pm

AndrewT wrote: Never heard of anyone with a Lib or wrx having that kind of issue even with some guys pushing rediculous housepower.

yeow housepower!

dont forget to get one of those super duper front nolathane diff mounts like brenny got. eliminated heaps of knocks

AndrewT · Post by **AndrewT** » Mon Jun 15, 2009 8:10 pm

Thanks for that Stamplicker. Altho that one too looks like it may transfer vibration to the body as the extra braces there don't appear to be mounted with rubber.

Subafury haha, yes, I am now the owner of that very same diff mount

discopotato03 · Post by **discopotato03** » Mon Jun 15, 2009 10:02 pm

When my old Bluebird was converted from live axle to semi trailing arm IRS the fabricator had to make a rear diff mount (moustache bar) from scratch to hold up the bum of its R200 LSD .

What they did was machine up a pair of cylindrical steel sections that the bushes fitted into . The bar itselt was a rather thick section of steel plate and the above mentioned sections were welded on either end .
The bushes were urethane and they fitted large rubber washers above and below the bar ends to stop it floating up and down . The washers were cut out of heavy duty mud flap material .

I guess a thing to watch is that you don't overload the body mounts because they would have been designed around having soft mushy bushings all round .

I reckon it would help if you fabricated a new front diff mount bracket and removed much of the fore/aft movement from its custard bush .
There may be the opportunity to try to stop the twisting or torque reaction into the diff from the tailshaft , its seems Subaru did this on the L Series by having that long lever (moust bar) across the back of the rear diff .
Datsun semi trailing arm rear ends have a rectangular hole through the crossmember to which their long nose R series diff bolts solidly . They have large diameter bushings at the X members outer ends so the mossie bar isn't trying to resist as much twisting action as an L Series is .

I think the answer probably lies in tying the nose of your diff more securely to your rear X member and making sure it's up to the task and not overloading its mounts/bushings .

Just my 2c , cheers A .

AndrewT · Post by **AndrewT** » Tue Jun 16, 2009 10:48 am

Yeah I have a front mount with a brand new nolathane bush which is nice and tight. I'll see how that holds up first and concentrate on customising the rear mount first I guess.

Any comments regarding mounting the diff to the body with no rubber mounts in between like in two of those examples? I'm concerned about transferring vibration to the body...

brumbyrunner · Post by **brumbyrunner** » Tue Jun 16, 2009 11:21 am

AndrewT wrote:Yeah I have a front mount with a brand new nolathane bush which is nice and tight. I'll see how that holds up first and concentrate on customising the rear mount first I guess.

Any comments regarding mounting the diff to the body with no rubber mounts in between like in two of those examples? I'm concerned about transferring vibration to the body...

I can't really comment on the NVH levels of solid mounting my rear diff because there are so many other mods. It may interest you to know that the front diff mount bush is the same as the two rear diff mount bushes.

Replacing all 3 with new ones and reinforcing the moustache bar should keep you going for a while.

discopotato03 · Post by **discopotato03** » Tue Jun 16, 2009 11:22 pm

You can guarantee that if you mount the diff solidly to the floor pan any noise that it/the rear uni/drive shafts make will be transmitted straight into the floor pan .

Your call but as I said would be considering making the cross member absorb more of the drive lines torque reaction because you'd think it would be more securely mounted into the shell than the rear or mossie bar .
Actually thinking about it you could probably kill two birds with the one stone by solidly mounting the nose of the diff to the cross member , with the front of the diff taking more of the torque reaction the rear bar and bushes get less so need less ironmongery added to them . Also with the cross member bolted to the diff it is going to get less movement because the diff becomes a load bearing member between the mossie bar and the cross member .

Do a search on Datsun 1600/180B/IRS 200B and have a look at how they do it . These are RWD cars that used long nose R160 differentials and they make similar or greater power than your average L series Subaru .

You can weld lots of scaffolding into the rear of your car if you like but I think its easier and adds less weight to use the cross member , or a stronger modified or completely fabbed one , where possible .

All of this a lot of work and if you can make your car AWD it goes a long way to solving many of these issues . AWD cars share the drive loads a lot more evenly than FrWD or RWD ones , when there is good traction they have 50/50torque split meaning each end is only taking half the overall drive reaction .
If you literally halve the torque reaction to the rear diff it won't be doing much of what it would in RWD mode .
My Aus spec 86 L RX-T was totally different to drive when it grew an AWD L transmission , it suddenly didn't hop and skip over speed humps because the rear end was doing something useful - taking half the load off the front diff/tyres .

IMO running a Subaru in RWD mode creates problems because the factory never intended their cars to work in that mode so didn't set the things up to cope with all the power going to the rear end .

A .

AndrewT · Post by **AndrewT** » Tue Jun 16, 2009 11:37 pm

Yeah I wouldn't normally run it RWD but it's getting a transfer case conversion and RWD is the way it works for normal on-road driving. I'll be able to engage the front wheels when I want but it will be locked in the centre so not suitable for on-road driving. Can't really afford an AWD transfer case (like some landcruisers etc have) and also I'm pretty sure there wouldn't be a divorced type of a suitable size for me to use either. It seems to work okay for the Datsuns etc which run a similar diff, only real difference for me is maybe abit more power from the EJ20 and obviously larger wheel diameter.

Mounting the front of the diff solidly to the K-frame might be an idea as the K frame is mounted via rubber mounts itself anyway, altho I would still loose the original rubber front diff mount as an additional vibration absorbtion point. Hopefully just replacing the original front mount bush with a brand new very tight nolathane one and majorly beefing the rear moustache bar will suffice.

I'll probably make small upgrades as I go then I will find out what ends up being strong enough without over-doing it.

thanks for all the suggestions and pics - very helpful!

daza · Post by **daza** » Wed Jun 17, 2009 8:28 am

Lada Nive transfer case is a divorced case Awd with a low low and a mechanical center diff lock.
Daza.

discopotato03 · Post by **discopotato03** » Wed Jun 17, 2009 9:05 am

Just another 2c worth about urethane bushings .

I was warned of the lack of flexibility many urethane bushes have so its important to think about where you use them .
My above mentioned Bluebird was converted from 4 to 5 link before it went IRS and its parallel links had urethane bushes which overstressed the body mounts which began to crack .

Out of time but briefly I think you need to make something a bit beefier than Subaru's flimsy coat hanger otherwise the compliance the urethane bush now doesn't have is highly likely to twist or break the mount itself .

Later , cheers A .

vincentvega · Post by **vincentvega** » Wed Jun 17, 2009 9:24 am

you are building a 6" lifted EJ20T monster truck. are you really concerned with NVH?

AndrewT · Post by **AndrewT** » Wed Jun 17, 2009 9:57 am

Of course! The nicer it is to live with, the better.

Don't see why the size of the lift blocks or the fact that it's an EJ20G should make much difference anyway.
The NVH is already perfectly fine (for an L series), but I just don't really want to adversely effect it unneccessarily when beefing up the rear diff mount.

Thanks for the suggestion on Lada transfer cases, I'll do abit more research on these.

YouTube · Post by **El_Freddo** » Fri Jun 19, 2009 12:40 pm

I thought the Lada Niva divorced transfer case had already been put forward to you before? I checked one out at a wreckers a few months ago - I reckon they'd do the job, they come with two levers - one for hi/low range and one to lock the centre diff. Unfortunatly at the time I did not look under the vehicle to see how big these things are physically.

About the rear diff: I've seen a setup that uses the factory diff protector with a Y frame off each side extending forward to the crossmember that are mounted with standard L series swing arm bushes and mounts (obviously cut from another L's rear end) - the mounts are used left on the right and vise versa. The Y arms were bolted to the diff protector front and rear with the rear part of the arm being the longest part that then meets up with the mounting point. This eliminated any road noise and stops busting rear CV's from torque twist of the rear diff. I saw this setup in Gee's Brumby - it was lowered and setup for street racing, it also had a very tight LSD in the rear... I believe Hamish71 may have this setup now if you're after some pics.

Cheers

Bennie